Monday, March 3, 2014

How High Availability works on vSphere 5.x

How High Availability works on vSphere 5.x

When vSphere HA is created in a ESX cluster, one of the host is automatically elected as the master host. The master host communicated with vCenter Server and monitors the state of all virtual machines & slave hosts. When one of the ESX host fails in the cluster, the master host must distinguish between a failed host and one that is in a network partition or that has become network isolated. The master host uses network and datastore heartbeating to determine the type of failure.

When a host is added to a vSphere HA cluster, an agent is uploaded to the host and configured to communicate with other agents in the cluster.

HA election process

1) All active hosts participates in the election to choose the cluster's master host
2) Hosts with more number of datastores has an advantage in the election
3) In a cluster, there could be only one master host.
4) If the master host fails (or) shut down (or) in standby mode (or) removed from cluster, a new election is held.

Master host responsibilities

1) Monitors the state of slave hosts.
2) If a slave host fails or becomes unreachable, the master host identifies which virtual machines need to be restarted.
3) Monitoring the power state of all protected virtual machines. 
4) If one virtual machine fails, the master host also determines where (on which host) the restart should be done.
5) Manages the lists of cluster hosts and protected virtual machines.
6) The master host has the list of protected virtual machines in the cluster's datastores.

Slave host responsibilities

1) The slave hosts contributes to the cluster by running virtual machines
2) Reports the state updates to the master host.
3) A master host can also run and monitor virtual machines. 

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